For the purpose of increasing in output of a semiconductor laser device, a type of semiconductor laser device has been proposed in which the degree of freedom in the energy band gaps of clad layers formed on the outer sides of carrier blocking layers is increased by providing the carrier blocking layers having a wide band gap and a small thickness on both sides of an active layer. In such a construction, the carrier blocking layers efficiently confine injected carriers into the active layer and since the carrier blocking layers are formed thin, light generated in the active layer can easily pass therethrough and leak out to the outer clad layers. This prevents instantaneous optical damage which is caused by intensive concentration of laser beam at the output end facets of the semiconductor laser device, and increases the COD (Catastrophic Optical Damage) level on the output end facets hence permitting a higher laser output.
FIG. 11(a) is a cross sectional view of such a semiconductor laser device, FIG. 11(b) is a profile of band gaps in their respective layers, and FIG. 11(c) is a profile of the effective index of refraction in case where the carrier blocking layers and the active layer are formed adequately thin so as to hardly affect the waveguide mode. The construction shown in FIG. 11 is called perfect SCH (PCT International Publication No. WO093/16513) in comparison with a known separate confinement heterostructure (SCH).
Referring to FIG. 11(a), formed on an n-GaAs semiconductor substrate (not shown) are, sequentially from lower, a second n-type clad layer (n-AlGaAs) 1, a first n-type clad layer (n-AlGaAs) 2, an n-type carrier blocking layer (n-AlGaAs) 3, an active layer (a GaAs/AlGaAs multi-quantum well layer) 4, a p-type carrier blocking layer (p-AlGaAs) 5, a first p-type clad layer (p-AlGaAs) 6, and a second p-type clad layer (p-AlGaAs) 7.
As shown in FIG. 11(b), the band gap in each of the carrier blocking layers 3 and 5 is greater in width than that in any of the active layer 4 and the clad layers 1, 2, 6, and 7, thus allowing injected carriers to be effectively confined in the active layer 4. Accordingly, the number of carriers which stimulates the laser oscillation will be increased hence improving the efficiency of laser oscillation.
When the carrier blocking layers and the active layer are thin enough to hardly affect the waveguide mode, an effective distribution of refractive index, as shown in FIG. 11(c), is of a slab waveguide structure in which the first n-type clad layer 2 to the first p-type clad layer 6 constitute a portion of high refractive index and each of the second n-type clad layer 1 and the second p-type clad layer 7 constitute portions of low refractive index. Accordingly, light generated in the active layer 4 propagates throughout the high refractive index portion, and as a consequence the peak intensity in the waveguide mode becomes low and hence an optical damage on the output end facets hardly occurs, whereby a high output semiconductor laser device can be realized.
In addition, there is reported an InGaAsP/InP semiconductor laser device of MQW-DCH (multi-quantum well-decoupled confinement heterostructure) provided with hole barrier layers (IEEE, Journal of Quantum Electronics, vol.29, No.6, June 1993, pp. 1596-1600).
In order to obtain a semiconductor laser device of high-output and high-efficiency, it is important to reduce the internal loss due to absorption of free carriers as well as to efficiently confine the injected carriers in the active layer.
In a perfect SCH semiconductor laser device, the injected carriers are successfully confined in the active layer by the carrier blocking layers which has a widest band gap among the layers and is adjacent to the active layer. Since this carrier blocking layer allows light to easily leak out to the clad layers, generally the carrier blocking layer is formed into a very thin layer having a thickness of 0.01 to 0.03 .mu.m in the thickness. In case where the doping concentration of the carrier blocking layer which is formed to have a wide band gap and be very thin is inadequate, depletion of the whole carrier blocking layer occurs, resulting in inadequate confinement of the carriers in the active layer. The carrier blocking layer is thus required for increasing the doping concentration by use of a dopant having a high doping efficiency and a low diffusivity. However, zinc, which is commonly used as a p-type dopant is an element which is easily diffused in bulk form. Accordingly, the diffusion length of zinc drastically exceeds the thickness of the carrier blocking layer, hence it is impossible to form a high doping concentration in the very thin carrier blocking layers.
The efficiency of the semiconductor laser device largely depends on levels of internal loss caused by absorption of free carriers. The free carrier absorption is controlled by the doping concentration of each layer where light is propagated. The higher the doping concentration, the more the internal loss increases. Accordingly the doping concentration of each layer where light is propagated is required to be lowered at a minimum essential level.
FIG. 12(a) illustrates a band gap distribution in an SCH semiconductor laser device and FIG. 12(b) shows a band gap distribution in a perfect-SCH semiconductor laser device, which show examples where the active layer comprises a quantum well layer and two barrier layers sandwiching the quantum well layer.
In the SCH shown in FIG. 12(a), the clad layers formed to have wide band gaps and large thicknesses confine injected carriers into the active layer. Although the carriers in the active layer is about to overflow toward the clad layers by thermal excitation, they are diffused back into the active layer in a certain probability due to the thick clad layers. Accordingly a high efficiency for confining the carriers in the active layer can be attained, however since the waveguide mode concentrates in the active layer, a high output operation may easily cause damages on the end facets.
In the perfect-SCH of FIG. 12(b), the injected carriers are confined into the active layer by the carrier blocking layers which is adjacent to the active layer and have a widest band gap among the layers. For the purpose of allowing light to easily leak out to the clad layers, the carrier blocking layer is formed generally so as to have a small thickness of 0.01 to 0.03 .mu.m. The waveguide mode is thus extended and improvement of COD level is achieved, resulting in a high output operation.
The carriers which have flowed over the carrier blocking layers are distributed in the first clad layers which have smaller band gaps than those of the carrier blocking layers, as shown in FIG. 12(b). In this case, if once some carriers have overflowed, the overflowed carriers are prevented from diffusing back into the active layer by the high potential barriers of the carrier blocking layers. Accordingly, in the perfect-SCH, the efficiency of confinement of the carriers into the active layer will be easily decreased, and therefore it is necessary to suppress the overflow of the carriers.
For the purpose, the band gaps in the carrier blocking layers are increased to enhance a carrier blocking function.
However a material usable for the carrier blocking layer has a limit in band gap. Particularly in a III-V semiconductor compound such as AlGaAs, even though a wide band gap type of material is used, the offset of conduction band does not increase because the band edges become indirect transition type.
Also, the effective mass of conductive electrons is small and when the electron quasi-Fermi level ascends as the carriers are injected, the overflow of electrons will hardly be negligible.